Wood, which is one type of material containing cellulose, has a high specific Young's modulus and a high specific strength, and processing thereof is very easy, so that it has been used as a material for dwellings, furniture, and the like, and furthermore, because it possesses superior acoustic characteristics, it has been used in soundboards of musical instruments.
Recently, the fact that wood possesses characteristics which are desirable with respect to the psychology, mental therapy, and health of human beings has been scientifically recognized, and the necessity of wooden material as a material for finishing the insides of dwellings has been recognized. Furthermore, wood is judged to have an elegance not present in concrete or metal, and has recently drawn attention as a structural member for large buildings.
However, while wood possesses these superior characteristics, depending on the environment and position of use, it is also susceptible to rotting, susceptible to dimensional variation, and the resistance to water and moisture thereof are inferior to those of other materials, and it is also susceptible to being eaten by termites and teredo and the like. In order to eliminate these types of defects associated with wood, it is necessary to improve the quality of the wood or to impart new characteristics to the wood, and the chemical modification of the wood is an extremely effective method therefor.
The chemical modification of materials containing cellulose such as wood is commonly conducted by replacing the hydroxyl groups present in the cell wall components of cells forming the material containing cellulose with other functional groups. One example of such chemical modification is the acetylation of materials containing cellulose.
Conventionally, methods such as those listed below were known as acetylation methods for materials containing cellulose.
(1) A method in which polyvalent metal halides are used as a catalyst and wooden material is acetylated by means of acetic anhydride (English Patent No. 579255). However, in this method, even under optimal conditions for the use of a mixture of acetic anhydride, acetic acid, and zinc chloride, processing is necessary for a long period, a period of 24 hours, at a temperature within a range of 38.degree.-50.degree. C.
(2) A method in which oven-desiccated thin wooden strips are treated using an acetylating medium containing acetic anhydride mixed with other components such as tertiary amines and acetone, and containing no moisture (U.S. Pat. No. 2,417,995). In this method, it is preferable to conduct the reaction in a vapor phase by means of a mixture of acetic anhydride and pyridine. However, in this acetylating method, the pyridine forms a complex which is difficult to recover, and when the reaction temperature is too high, the pyridine darkens the wooden strips, while when the reaction temperature is too low, the reaction period is relatively long, and moreover, it is necessary to use harmful or combustible chemicals.
(3) A method in which wooden material is acetylated without catalyst, and using a combination of acetic anhydride and xylene at a temperature of 105.degree. C. and at an absolute pressure within a range of 150-170 psi (1.0-1.2 MPa) (U.S. Pat. No. 3,094,431). In this method, as a catalyst is not used, special treatment is necessary, and furthermore, the use of volatile, combustible organic solvents is unavoidable, so that the recovery of excess reaction products or byproducts is complicated.
(4) A method in which the acetylation of a hard board containing aluminum sulfate (functioning as a catalyst) is carried out in a vapor phase of acetic anhydride (TAPPI, Vol. 49, No. 1, 1966). However, in this method, it is not merely the case that the use of the aluminum sulfate is unavoidable, but it was also necessary to provide an extremely long exposure period (overnight heating).
(5) A treatment method is also known for lignocellulose materials in which, in order to improve upon the above methods, catalysts and solvents are not employed (Japanese Patent Application, First Publication, Laid-Open No. Sho 62-64501). In this method, the material is first immersed in advance in an acetic anhydride solution, then excess acetic anhydride is removed, and treatment was conducted for a period of 2 to 8 hours at a temperature of 120.degree. C. It is thus possible to conduct acetylation in a shorter period of time than was possible with the acetylation methods described above. However, in this method, as a result of constraints for a guarantee of quality of the products, it is impossible to raise the treatment temperature to a level of 120.degree. C. or more, so that there is a limit to the shortening of the treatment period. Furthermore, prior to acetylation, the material was immersed in acetic anhydride, and after this, excess acetic anhydride solution is removed, so that extra processes were necessitated.
As described above, the conventional acetylation methods for material containing cellulose have various problems, and it was thus either impossible to execute such methods on an industrial scale, or, even if it is possible to execute these methods on an industrial scale, there are problems in that manufacturing costs are high as a result of the long reaction time, the complexity of the processes, or the like.
Furthermore, in the conventional acetylation methods conducted in a liquid phase, even if post-processing such as washing with water or the like is conducted, there is a persistent acetic acid odor associated with the acetylated material containing cellulose, and metal materials which came into contact with the material containing cellulose, such as nails or the like, are likely to develop rust.